1. Field of the Invention
The present invention relates to RF power amplifiers and methods of amplifying an RF signal. More particularly, the present invention relates to feed forward amplifiers and related methods.
2. Description of the Prior Art and Related Information
Linear RF power amplifiers are designed to amplify incident RF signals without adding unwanted distortion products, producing output signals at significantly higher output levels. As is known in the art, amplifiers have a wide variety of applications. Amplifiers can be biased to operate in one of a number of so-called Classes. When biased to operate in Class A, the amplifier provides a linear relationship between input voltage and output voltage. While operation in Class A has a wide range of applications, when higher power output and efficiency are required or desired, the amplifier is sometimes biased to operate in Class A/B. When biased to operate in Class A/B, however, the Class A/B amplifier power transfer curve 25 is less linear than for Class A amplifiers, illustrated in FIG. 1 by trace 15. To increase efficiency, communication systems often operate amplifiers in the non-linear region 25. This practice, however, does introduce amplitude and phase distortion components into the output signal produced by the amplifier.
As is also known in the art, most communication systems have government allocated frequency bandwidths 18 (that is, in-band frequencies) centered about a carrier frequency Fc as shown in FIG. 2. For example, a CDMA (Code Division Multiple Access) IS-95 communication system signal has a predefined bandwidth of 1.25 MHz. Different CDMA communication channels are allocated different bands of the frequency spectrum. Amplifiers are used in such systems, and are frequently biased to operate in Class A/B. Referring to FIG. 1, signal processing such as amplification by an amplifier operating in the non-linear region 25 can produce distortion frequency “shoulders” 22a-22b outside a signal's allocated bandwidth 18 (FIG. 3). (These are called out-of-band frequencies.) These distortion frequency components 22a-22b can interfere with bandwidths allocated to other communication signals. Thus, regulatory bodies around the world impose strict limitations on out-of-band frequency components. Many techniques exist to reduce out-of-band distortion. One such technique is shown in FIG. 4 where a predistortion unit 24 is fed by a signal 22 to be amplified. The predistortion unit 24 has a power transfer characteristic 23 (FIG. 1) and compensates for distortion introduced by subsequent amplification in Class A/B amplifier 26. More particularly, the predistortion unit 24 transforms electrical characteristics (for example, gain and phase) of the input signal such that subsequent amplification provides linear amplification to the phase and frequency characteristics of the input signal. In one embodiment, compensation is effected by changing bias parameters of the predistortion amplifier or the main Class A/B amplifier. One method is described in U.S. Pat. No. 6,046,635 titled DYNAMIC PREDISTORTION COMPENSATION FOR A POWER AMPLIFIER, the contents of which are incorporated herein, in their entirety, by reference. The predistortion unit 24 is configured with a priori measurements of the non-linear characteristics of the Class A/B amplifier. Unfortunately, the amplifier characteristics (amplification curve 10 with region 25 in FIG. 1) change over time and temperature making effective predistortion more difficult. For example, as the temperature of the amplifier increases, its non-linear region 25 may become more or less linear, requiring a compensating change in the transform performed by a predistortion unit 24. As shown in FIG. 6, some adaptive predistortion systems use a control system 30 to alter predistorter characteristics based on environmental factors such as temperature. Typically the control system employs a look-up table with predetermined predistorter control settings for use in predetermined situations. However, environmental factors alone do not determine the alterations in an amplifier's characteristics. Thus, over time, amplifier characteristics vary unpredictably due to aging of amplifier components.
Another approach to reduce amplifier distortion is to use feed forward compensation, as shown in FIG. 5. Here, a feed forward network 100 is included for reducing out-of-band distortion. The feed forward amplifier includes a differencing network or combiner 25, a main amplifier 20 operating as a Class A/B amplifier, an error amplifier 35, delay circuits 15 and 30, and a combiner 40. The differencing network 25 produces an output signal representative of the difference between a portion of the amplified signal output from the main amplifier 20 operated in Class A/B and the signal fed to the amplifier 20 prior to such amplification. The frequency components in the differencing network 25 output signals are, therefore, the out-of-band frequency components 22a-22b introduced by the main amplifier 20 as illustrated in FIG. 5. The output produced by the differencing network 25 is amplified by error amplifier 35 to produce an out-of-band correcting signal. The combiner 40 combines the correcting signal produced by differencing network 25 and error amplifier 35, 180 degrees out of phase with the delayed signal (delayed by delay 30) output of the main amplifier 20 thus reducing the energy in the out-of-band frequencies 22a-22b of the signal output by the main amplifier 20. Feed forward network 100 includes delay line 30 to compensate for the delay in error amplifier 35. It should be noted that minute differences in timing between these elements can impair the effectiveness of a feed forward system. While a manufacturer can carefully match components prior to shipment, as feed forward components age, the correcting signal and processed signal can become mistimed if not properly compensated. This will limit the ability to cancel the out-of-band distortion. Another problem associated with the delay line 30 is that significant output power losses occur for delays which are long enough to match that of the error amplifier path. These losses are highly undesirable.
Accordingly, a need presently exists for a system and method for amplifying RF signals while minimizing power losses and minimizing out-of-band distortion.